I know it's selfish, but...
I would like rdoc to have the ability to build a landing page/portal  
locally accessible in a browser.
That way there would be quick, easy to navigate docs available from  
one aggregate place.
In addition to this, the ability to configure it to rebuild at  
intervals/events. For example, after a gem is installed, or every  
Monday morning at 3:00 would be convenient. Ideally, this would be  
daemonized and maybe simply take advantage of platform-available  
tools (such as chron on *nix's and whatever is available on Windows).